text:
How are brakes cooled on heavy aircraft?
As kinetic energy is not stored or recycled, large quantities of heat have to be evacuated in a short period of time when aircraft abort their takeoff or land:

B777 brake test, Youtube
Which approximate quantity of energy is transferred to the brakes in a rejected takeoff for heavy aircraft, e.g. Boeing 777 or Airbus A380? What are the typical systems used to cool brakes on these heavy aircraft? Is cooled air or liquid used, or just ram air?

One set of brakes on an A380 converting kinetic energy into heat at 5 MW during a test: https://www.youtube.com/watch?v=m1dv_y_3EK0

The amazing thing in that video is that a 5MW dynamometer exists

Summary
The order of magnitude of brake energy after a rejected takeoff (RTO) for a very heavy aircraft is 1 GJ, or 100 MJ per wheel brake.
Aircraft brakes are cooled with ambient air, either through passive airflow or (optionally) through forced air ventilation.

Cutaway drawings of A380 wheel and brake (Source)

Energy Dissipation
Takeoff speed for heavy aircraft is about 150 knots or 280 km/h. The maximum speed to reject takeoff ($V_1$) depends on the plane, the load, the wind, and the condition of the runway. Let's assume 150 knots for the sake of a rough energy estimation.
The maximum energy that could be transferred to the brakes is more or less the kinetic energy of the plane at takeoff speed:
$$E_\rm{kinetic} = \frac{1}{2}mv^2 $$
The actual energy dissipation will be increased by

remaining power delivered by engines
tail wind
runway downward slope

and decreased by

airbrakes
reverse thrust
head wind
runway upward slope
drag
tire heat dissipation and tire wear

Neither A380 nor B777 have brakes in the nose gear. (Actually very few heavy aircraft have braked nose wheels. See: Are there any aircraft with a nose wheel braking system?)
Airbus A380
The maximum takeoff weight (MTOW) of the A380 is 575 t, and its main gear has 20 wheels. 16 main wheels have brakes and 4 main wheels do not. (See Why do some A380 main wheels have no brakes?)
$$E_\rm{TO,max} = 0.5\cdot 5.75\cdot 10^5\,\rm{kg}\cdot\left(\frac{280}{3.6}\frac{\rm{m}}{\rm{s}}\right)^2 = 1.7\,\rm{GJ}$$
The approximate maximum dissipated energy per wheel brake unit for an A380 after RTO is
$$E_{\rm{wheel}} = \frac{1}{16} E_\rm{TO,max} = 110\,\rm{MJ} $$
Boeing 777
The MTOW of the heaviest 777 is 352 t, and its main gear has 12 wheels.
$$E_\rm{TO,max} = 0.5\cdot 3.52\cdot 10^5\,\rm{kg}\cdot\left(\frac{280}{3.6}\frac{\rm{m}}{\rm{s}}\right)^2 = 1.1\,\rm{GJ}$$
The approximate maximum dissipated energy per wheel brake unit for a Boeing 777 after RTO is
$$E_{\rm{wheel}} = \frac{1}{12} E_\rm{TO,max} = 89\,\rm{MJ} $$
Active Cooling
Aircraft which are foreseen to take-off and land frequently may be equipped with electric brake cooling fans (BCF) which extract the interior hot air to the exterior. The BCF are used during ground time to shorten the turn-around time.
As an example, the main wheels of an A318 equipped with electric brake cooling fans:

Interestingly, there is a difference between steel and carbon brake disks. Steel disks wear out faster when hot, whereas carbon disks wear out faster when cold. Therefore it is preferable to reduce the cooling of carbon disk brakes to what is required for the following takeoff (see this answer).
Passive Cooling
Many (long range) aircraft have no active brake cooling. Passive cooling is caused by

airstream (induced by wind or aircraft motion),
convection,
thermal conduction,
thermal radiation.

Passive cooling of aircraft brakes follows Newton's law of cooling, which is - in terms of mathematics - an exponential decay (or exponential function with a negative exponent, if you prefer).
It may take many hours of passive cooling to reach the ambient temperature. Also see related question What is the temperature of the brakes after a typical landing?

The source of this graph does not indicate the unit of the x-axis, but minutes seem most plausible. The type of the aircraft under test is also unknown. The information is applicable to a range of Airbus aircraft equipped with carbon disk brakes, from A300 to A380.
External Cooling
External ventilation units exist and may be used whenever necessary. These can be electric, petrol powered,

or supplied by a remote compressor or extractor:

Both of the answers in this thread so far are simply outstanding in their thoroughness. Very well done!

Sorry for the possibly silly question, but I'm no expert. The unit on the horizontal axis of the graph in the "passive cooling" section is minutes or hours? From the text on the graph and common sense I would assume minutes, but you talk about "many hours" in the previous text (or maybe I didn't understand the assumptions).

@LorenzoDonati Not silly at all. The x-axis is unlabeled in the source document, but minutes are plausbile. The first landing is at 70'. The cooling follows the law of exponential decay. If there was no 2nd takeoff, it would take approx. 2h to cool down to 100 dC (somewhere after 200'), and more hours to get down to ambient temperature.

@LorenzoDonati I added a caption to address your question.

@LorenzoDonati Has to be minutes.  No ordinary aircraft could stay aloft for that many hours.

@LorenPechtel Yep, that's what I thought, too. But I'm quite incompetent in the subject, so I didn't feel confident enough to rule that out altogether.

Hot brakes are normally cooled by natural convection, but in extreme cases the fire fighters will cool overheated brakes. Some designs use brake fans. From the linked page:

Brake fans reduce brake cooling times by using wheel mounted electric
  fans to blow ambient air across the brake and wheel assemblies. Note
  that the maximum recommended temperature for takeoff as indicated on
  the instrument panel may have a different value dependent on if the
  brake fans have been used or not.

Safran brake cooling fan and electric motor (picture source)
According to this page on Airliners.net, the BAe 146 had them, as did PanAm's A310s, but most airlines leave them off to save some weight. Short-haul aircraft make more use of cooling fans: According to this article and video, 90% of A320 use cooling fans. There are also ground-based cooling fans to shorten turn-around times.
The most severe case used in the design is a rejected take-off. Some energy is absorbed by rubbing off the tires, but most ends up in the brake disks. To absorb all this energy, sometimes more disks have to be added in the design phase. The most extreme example is the landing gear of the Tupolev 144 which sported the unusual combination of a high take-off and landing speed and no thrust reversers.

Tupolev Tu-144 landing gear (picture source) Yes, it is this bird!
As always, not all use cases can be foreseen by the designers. The reason why aviation hydraulic fluid must be non-flammable is one case when the heat energy was absorbed by the surrounding structure, with fatal consequences. In this particular case in 1963, a Sud Aviation Caravelle had tried to heat away fog on the runway so they could take off. They did this by taxiing down the runway with considerable thrust and used the brakes to keep the aircraft from accelerating. When the gear was retracted, the brake disks were still glowing red but had little to no air cooling. So the heat dissipated into the aluminium wheel hubs, which became too soft to withstand the tire pressure. Shrapnel from the burst wheels cut hydraulic lines, and the still glowing brake disks ignited the oil which was pumped into the wheel well. The resulting fire destroyed the aircraft.
From the accident report:

Four minutes later witnesses on the ground noticed a whitish trail of
  smoke on the left side of the aircraft and suddenly a long flame from
  the left wing-root. Around 0620 hours the aircraft reached an altitude
  of approximately 2700 m, it then began to lose height, entered a
  gentle left turn loosing height more rapidly and finally went into a
  steep dive.

This is why dragging brakes on retractable gears are dangerous: After take-off the wheel and brake is retracted, so air cannot cool it down anymore.

Peter, thanks for these interesting facts. I wish I could select two answers.

@mins: You selected the right answer. I never calculated the energy and just added some lore. Especially the Tu-144 which ended up in Tatarstan never ceases to amaze me.

Great answer! I noticed a bug though: the link in "From the accident report:" points to the photograph of the Tu-144 landing gear, not to the accident report.

@KevinDuggan Thank you very much - this happened because I manually edited the link and later add more before. Fixed.

? If it's a safety issue that hot brakes sometimes do need to be cooled after being retracted, then it seems plausible for there to be openable venting on the underside of the aircraft, so that some air is either rammed through the wheel compartment or on-brake electric fans can run and push the heat outside without affecting thrust.

@DaleMahalko: It's much better to have temperature probes and delay retraction a little. And it's best to keep the plane in shape so dragging brakes don't happen.

Aircraft hydraulic fluids are still flammable; they just have to be heated a lot more than older hydraulic fluids did before they ignite.

As others have answered, typically brakes are cooled by natural convection.  For single-aisle and especially regional aircraft, these are designed to cool to taxi-out temperatures in the time it takes to deboard and reboard the aircraft.  For certain aircraft, cooling may be the determining factor in when aircraft depart the gate, which is where temperature management schemes come into play.
Operators have two options when it comes to temperature management.  The first is a chart published by the airframer, which is used to estimate the cooling time based on the landing energy and ambient conditions.  However, this chart is usually conservative due to uneven heat distribution in brakes, so most operators choose the optional Brake Temperature Monitoring System upgrade.  This option uses a temperature sensor called a  thermocouple to determine the actual temperature of the brake linings (sometimes called the heatsink).  This is integrated with the Engine Indicating and Crew Alerting System (EICAS) so the pilots know when it is acceptable to taxi out.
The consequences of excessive heat in brakes have already been mentioned.  After the Sud Aviation incident, the industry introduced fuse plugs into aircraft wheels.  These contain a eutectic material which melts and releases tire pressure prior to wheel rupture.  Aircraft tires are inflated with nitrogen, the release of which suppresses any fire which might occur.
For those interested in what happens when fuse plugs do not operate properly, here is a video:

You're welcome.  Not sure about which airport this is.  I used to work for the company that manufactured the brakes, but it happened before I joined on.

The video is no longer available.


text:
What is the technical role of a flight attendant?
Everyone knows flight attendants represent the airline for the passengers, and have a commercial role by distributing meals, selling goods, managing the entertainment system, and receiving complaints.

One of the well known glamour airline-clichés for flight attendants, circa 1960 (source).
Apart from this commercial role, they also provide the regulatory information about safety. But I feel there is more to know related to their technical role.
What are the non commercial missions or responsibilities of a flight attendant, what are they accountable for? did that role change over years? How are they trained? What is their career path?

Managing passengers in an emergency, and evacuation comes to mind. To me the safety aspects of the cabin crew is the most valuable, while simultaneously the most underrated by the general public, which is a shame

Is there really a need to put stock photos in questions? Are we turning into Medium?

Their regulatory-mandated role is as safety technicians. The FAA expects them to manage passengers in the event of emergencies, whether those emergencies consist of passengers suffering medical events, being disruptive, or having to evacuate a burning airplane. Their hospitality and customer service roles are actually secondary to the role they play in ensuring safe and successful completion of their flights.

There primary role is safety.  Everything is just added sugar.

@mins Because it's not relevant to the question. It doesn't aid in understanding, or support the content. It's just like a more space-hungry version of the salutations new users sometimes add to their questions.

@Simon Actually I thought the image did help, because it drew attention to one aspect of the question that I think is interesting and will inevitably be neglected: how has their role changed?

@DanieleProcida Feel free to reverse the edit then.

@Simon: I rolled back, and added a legend so that so that everyone can understand the meaning of the picture. This profession has been one with a lot of discriminatory selection criteria (female, single, features, etc) that were declared illegal one by one (unfortunately the old cliché is still in the imagination of some customers). This picture is a reminder. Thanks to Daniele.

I flew for a major airline as a flight attendant for 7 years.  The FAA requires flight attendants on board for the safety aspect of passenger service and sets the minimum number based on the aircraft's passenger capacity.  We usually flew with at least one more crew member than the minimum required by the FAA.
Our FAA mandated duties include safe boarding, including stowage of personal articles, arming the evacuation slides after the doors are closed, a safety briefing prior to departure, verification that the cabin is safe for flight, inflight enforcement of pilot instructions (usually return to seats and fasten seat belts), and to render passenger assistance as necessary for flight safety.  Same for pre-landing and de-planing.  First aid training (including how to deliver a baby in-flight) was rarely seen by the public.
Annually we would be required to attend jet recurrent training where we reviewed NTSB accident reports with an emphasis on cabin crew responses to emergencies, review of first aid and CPR techniques, re-testing of our knowledge of the location and how to use all the safety equipment on board, including life preservers, life rafts, and emergency slide procedures.  We were also briefed on any changes to FAA mandated procedures and tested on everything.
We were trained to be current on every aircraft the company flew so that we had maximum flexibility for our scheduling.  At the time I flew, this would have been B-727, B-707,B-757, B-767, DC-9, DC-9 stretch, and L-1011.  That meant we had to know the location of every flashlight, first aid kit, supplemental oxygen bottle, safety briefing kit, life raft, life preserver, safety exit, safety lighting, etc. for every aircraft type we flew.
The normal passenger service duties that passengers see such as food and beverage services, in-flight entertainment and generally trying to be accommodating, were in addition to the FAA required duties and were entirely up to the company's discretion to provide and train us for.
The FAA mandated safety aspects of flight attendant duties have become so integrated with the normal flow of a commercial flight they can go nearly unnoticed, especially for routine travelers.
And no, we weren't trained how to land a plane in case all the pilots got sick at the same time and passed out.

Thanks for this interesting answer. I was not aware flight attendants were trained to deliver a baby. As @falstro already commented, FA role is sometimes misunderstood. Is it possible to know about a FA team structure? Do you know if the FA role is similar under other regulation agencies?

FA team structure is based on a monthly schedule via a seniority based bidding system.  For any given flight, a Lead Flight Attendant was assigned, usually based on this bidding.  The Lead is responsible for coordinating with scheduling, the flight crew, the ground crew, catering, etc.  Cabin assignments were done on a rotating basis unless, by agreement, a couple of FA's really preferred one cabin over another in which case they could swap - for the entire month's schedule sometimes.  The Lead wasn't necessarily the most senior, just the most senior who wanted that position for the month.

In the FAA word, 14 CFR 121 has a lot to say about flight attendant responsibilities. In fact the term flight attendant is used in Part 121 150 times. The most direct sections of Part 121 are 121.391 Flight attendants, 121.392 Personnel identified as flight attendants, 121.393 Crewmember requirements at stops where passengers remain on board, 121.394 Flight attendant requirements during passenger boarding and deplaning and 121.397 Emergency and emergency evacuation duties.
There are more sections dedicated to training and duty times as well, and various other responsibilities are sprinkled throughout Part 121.
Section 121.391 basically requires at least 1 flight attendant for each full or partial block of 50 seats. This is why when American Airlines went to a 160 seat configuration on their 737-800s they had to add a fourth FA. Later they removed ten seats so they could eliminate the extra FA.

The elements in the answer seem very complementary to the other answer, and the references valuable, however the answer would be better is you could summarize the important points in addition of providing the links.

@mins I think PJNoes has done a great job of summarizing everything as much as possible, so I don't think I could add to that, except by providing the regulations that require those responsibilities.

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text:
Is a firm landing an indication of the pilot inexperience?
In a commercial airplane, when a landing has been a bit firm, a passenger may think it was not perfect, or maybe performed by an inexperienced pilot.

(Source)
A perfect landing is seen by passengers as one where the contact with the runway is almost imperceptible. Is that true? are there other technical or regulatory explanations?

In a crosswind you want a rather firm landing to minimize the side skidding. Or so I've been told.

Same for wet surfaces, to avoid hydroplaning. Or so I've been told. :)

For the shortest possible landing distance a firm landing without a long flare enables the pilot to engage the brakes earlier. A soft landing is a luxury for long runways, empty airplanes and strong headwinds.

Sometimes where the experience pilot lands hard, the inexperience one would have crashed, so you can't generalize that.

To quote John Finnemore's Cabin Pressure: MARTIN: For your information, a firm landing is generally the safest. CAROLYN: If that landing had been any safer it would've killed us.

@ymb1: there's a reason the flight deck is armored... :-)

Speaking as a passenger, in poor conditions when flying near the ground is difficult, I'd far rather the pilot slammed it down good and hard rather than floating gently in waiting for a nasty gust to upset the applecart.

"Did we land or were we shot down?"

First of all, the landing should be in the touchdown zone. Often I see pilots try to achieve a very smooth landing but floating far out of the touchdown zone before touching terra firma. Then they have to brake hard and taxi of at the far end. There are no points for landing outside the touchdown zone, even if it is the smoothest landing imaginable. In many overruns, landing late is a contributing factor.
A good landing is in the touchdown zone, firm enough to quickly spin up the wheels (and prevent wear by smearing rubber over hundreds of feet), and not so hard that it bounces up again. In calm winds, that can be very smooth, cross wind and wet / contaminated runways require a bit more of firm landing.
Hard landings1 can occur due to turbulence (wind shifts), upslope of the runway or when the air density above the runway is lower due to heating (typically in calm winds, with sun heating the runway surface)
So a hard landing is not necessarily an indication of pilot inexperience. However, some of the hardest landing I have ever experienced were when I joined in a training flight where new airline pilots were practicing landings in a Boeing 737 for the first time after their simulator training. I wouldn't be surprised if the runway was dented :-)

1Hard landing is used as a colloquial term here, not in the context of EASA CS 25 §473. See this answer for that context.

Does a fast spinning up of the wheels cause less wearing then a longer and smoother one? The work done by the friction force must be the same, but the in the latter case the stress (temperature rise, ...) looks milder to me. Isn't this like the wearing of the brake pads when going very hard as opposed to smother?

@DarioP I believe the wheels are being spun up by contact with the ground - so there's not really a way to spin them up gradually, your choices are "scrape rubber off as you skid along the ground" or "spin them up fast".

@DarioP Not all work is done by friction. In case of the faster spin up, elastic deformation of the tire ensures the scraping is reduced. Still, you end up with some of a flat spot.

Spinning the wheels up is a farse you can't flat spot tires from landing too lightly unless you press the brakes too soon. https://aviation.stackexchange.com/questions/3702/why-are-aircraft-tires-not-pre-spun-prior-to-landing-to-preserve-them

A perfect landing is seen as one where the contact with the runway is almost imperceptible.

No, the perfect landing is firm. The aircraft should touch down with a not strong, but still perceptible jolt. The reasons are:

Weaker initial contact between wheels and surface means the wheels take longer to spin up. The longer skidding abrades more rubber and on a wet runway the plane may start hydroplaning. So especially on contaminated (wet, icy or snowy) runways, too soft a landing must be avoided.
Making a softer landing means landing at a lower angle, which extends the distance still in the air, which might then be missing at the end of the runway. So on shorter runways, a very soft landing increases the risk of overrun.

This is generally true for large, heavy aircraft landing on hard, prepared surfaces, but will not necessarily be true for lighter aircraft, or aircraft landing on some types of soft surfaces.

@JonathanWalters the wheels take longer to spin up I've always wondered if the wheels are spinning before touchdown.  And if not, why.  Can the manufacturers put little wings on the wheels to get them spinning via airflow?  Is that a bad idea for some reason?

@user2023861, pre-spinning the wheels has been tried and found not to be worth the trouble. See http://aviation.stackexchange.com/q/3702/524.

If you use proper crosswind technique you don't need to be extra firm to prevent wear on the tires...

are there other technical or regulatory explanations?

A few points to add to the excellent answers already given:
Different aircraft models vary widely insofar as getting smooth landings. If you fly a lot, keep track of the aircraft type and what kind of landing you experience. Over time and many landings, you may see a difference.
In my own experience as a pilot, I found the Boeing 727-100 to be the hardest aircraft in which to get consistently good landings. Actually, I was never able to get consistently good landings in the 727. A string of, say, four landings in a row that could reasonably be described as "smooth" was rare. The joke among our pilots was that for the last six vertical feet before touchdown, it was the airplane, not the pilot, that was deciding what kind of landing it would be.
On the other hand, the easiest aircraft to get smooth landings in for me was the Boeing 747-100/200. You rarely got a landing that wouldn't be described as a good landing. Landings in which the first indication that you were down was the deployment of the speed brakes were not uncommon. When the mains were on, the speed brake handle in the cockpit would very noisily come out of its detent and travel all the way aft.
The pilot's landing currency also plays a role. A 747 pilot's day is one leg, one landing. If the pilots are trading legs, you're only going to get a landing every three days or so, maybe less. Commuter pilots on the other hand can get multiple landings every flying day. We tend to be better at doing that which we do often.
It doesn't often occur, but an aircraft configuration different than that which a pilot is used to can also affect the quality of the landing. For example, when they load 747-100/200/400 freighters they aim for a zero fuel weight c.g. of 26.6% MAC. One day due to the unique load we were carrying, the c.g. was up against the forward limits, 13% for both the zero fuel and landing c.g.s. The difference between 13% and 26.6% translates to a little over 44 inches. That meant that once the mains were down, the nose would want to fall through much more quickly than normal. I had forgotten about the forward c.g. and failed to use the necessary up elevator. The nose gear came down on the runway really hard, so hard I embarrassingly asked the flight engineer (an A&P) to inspect it.

Interesting answer Terry. If I could vote twice, I would do so, I hope other will do for me, so that your post is moved up in the stack and well visible!

As it relates to large aircraft, a hard landing means something very specific, defined in EASA CS 25 §473 and in its FAR equivalent, which is an event in which the aircraft design parameters may have been exceeded with compromise to its structural integrity.
The event may be reported by the pilots and/or detected by the aircraft's ECAM / AHM and possibly transmitted via ACARS. In any case, a thorough inspection will follow along with an occurrence reporting. Where deemed necessary, an internal or external investigation may be opened.
As to the causes, they are myriad. I am not going to speculate on their relative probabilities but anyone interested is free to trawl over the incident reports published by the accident investigation boards of his choice and come to their own conclusions.
In any event, in aviation a landing is either hard or not, there is no such thing as "a bit hard" (well, except on some pilots' tech log entries). There is also a number of other abnormal landing conditions which are just as if not more serious, such as nose-wheel landings, crabbed landings, etc.

A perfect landing is seen as one where the contact with the runway is almost imperceptible.

By whom? It is imperative, for reasons that have already been explained in other answers, that positive contact with the runway be achieved. Smooth landings are suboptimal and end up as a contributory factor to runway overruns.
From a passenger cabin point of view, you will know if you've experienced a hard landing by checking the weather: if it's raining luggage, it probably counts.

Good point about the official use of the term hard landing. I've updated my answer.

.. and: welcome to Aviation.SE!

As currently written, it explains more what to do when the aircraft is damaged than whether a moderately firm landing should be interpreted by a casual passenger as the mark of an inexperienced pilot. I've edited the question to replace hard by firm. Thanks.

I think you are confusing "hard" with "firm". A hard landing is one where the airplane needs a full inspection and possibly repairs, a firm landing is where the airplane is fine but the pilot's pride is damaged.
A simple way of explaining what happens in landing is that you reduce the airspeed of the airplane to the point where the wings no longer produce enough lift to keep the airplane off the ground, and therefore it settles onto the runway. If you hold the airplane too far up during the flare then it will drop much further when this point is reached, and the landing will be firmer and you may get a bounce or two. A sudden, powerful downdraft of wind when you are in the flare can drop you onto the runway surface before you intend as well.
Good landings take practice but nobody is perfect, and even very experienced pilots make jaw-juddering arrivals occasionally. Less experienced pilots will tend to make more of these than more experienced pilots, but it can happen to anyone.

Not necessarily.  A 'hard' landing being a relative term can be due to a number of factors, some outside of the pilot's control.
Landing technique is going to vary depending on aircraft type, the type of landing gear fitted to the a/c, the runway conditions and local weather, specifically windspeed and direction.  A 747 captain may 'grease' a properly executed landing while an F-18 pilot making a pass at the boat will slam it into the deck at 700 feet/min for a good landing as well.
And also good pilots mess up a landing every now and then.

A firm landing is no more an indication of pilot inexperience than rotation velocity. Different airframes have different flight characteristics, such as jets which are "Slick" or "Clean" - and as a result of this, they must literally be flown onto the runway with drastically less feathering than "Dirty" airframes such as single engine land airplanes. There are many factors which contribute to the impact moment experienced when contacting the tarmac, to include: Deflection due to flap positioning, characteristics of the airframe, speed of approach, glideslope, headwind, tailwind, quartering headwind or tailwind, whether the pilot chooses to crab into the runway or adjust his angle of bank, wind velocity at the surface, variability in wind velocity at the surface of the tarmac, variability in ground effect lift present, presence of wingtip vortices from preceding aircraft, the amount air pressure in the tires, the flexion and suspension travel in the suspension system for the landing gear (varies based on intended load), the density altitude at field level, thermal updrafts from the asphalt, any cratering or potholes in the runway, the powerplant/engine being used (Reciprocating engines are much more responsive to adjustments in rpm/power than jet engines which "Lag" input stimuli - therefore landings in reciprocating engined airframes can be softened by application of power with flaps dirty), etc. I could go on for hours - but it is important to realize: Any landing you (and the plane) walk away from is a good one. This is the great gulf of disparity between a human pilot and a Drone Operator - experience, skill, judgement, knowledge, and risk management.

Welcome! "Any landing you (and the plane) walk away from is a good one": The question is more about shades in good landings than the difference between good and bad landings.

Many airline pilots are ex-military.  Landing style may be indicative of their previous experience.  Navy pilots are trained to stick a landing more firmly than Air Force pilots.  Such preferences may be in addition to the other technical factors cited by more experienced pilots above.

A bad landing can be due to conditions or just random factors. Even the best pilots have rocky landings, it's just they get a lot fewer of them than inexperienced pilots.
In general, though, it is more likely that a bad landing is due to inexperience or lack of skill in the pilot.
Sometimes if I notice an older, experienced captain with a much younger first officer on a commercial flight and we have a rocky landing, I think to myself, "Guess he let the co-pilot land the plane."
On an old-timey note, believe it or not, in the 1970s it was a common practice for passengers to applaud after a good landing.

"In the 1970s it was a common practice for passengers to applaud after a good landing." Well I don't know which part of the world you live in, but in Europe it's still often the case.

Related to the point on applauding landings: When and why do people clap/applause after a plane lands? on [travel.se].

I think that the landing has more to do with pilot preference.The pilot is supposed to land within a "touchdown zone" (as mentioned above) and how hard the landing is really just depends on what the pilot prefers. (most of the time anyway...) When pilots try to make light landings for the passengers they end often end up outside their preferred comfort zone and end up landing wrong anyway.
Based on that info I believe that a firm landing isn't a sign of pilot inexperience.

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text:
Is it legal to board a plane dressed like a flight crew member if you are not a crew member?
Just out of curiosity: Are flight crew suits reserved to certain persons or can just be worn freely, provided one doesn't use company logos or other protected brand markings. If the country does matter, then in UK.

(Picture source)
Are the insignia like golden stripes or cap regulated by a public authority? Now questions of security may also be invoked by the Captain, or dictated by airline's rules, but that's not my main point, though explanations are welcome.

It is hard to cite regulation for this (because I can't find any forbidding it, so its like proving a negative), but it is legal in most countries including the US and UK. I could provide some online "testimonials" from air crew, but in the spirit of factual answers I'm not sure that would fit...

Sometimes it is much more effective to wear exotic costumes.

I've never heard of a law that prevents wearing a brand marking, either? o.o

@mins Huh, i know reproduction and distribution are controlled by IP laws, didn't know about wearing

@mins ok, i should have said creating and designing things also. I didn't think finding already existing articles and putting them on your body was an IP issue is all. I didn't think this post was talking about forging uniforms.

In the US, pilots of one carrier may end up flying another carrier to position themselves. This is not the preferred routing of pilots for economic reasons, but it happens. In that case there may be crew on board wearing a different carrier's uniform.
I know of no regulation prohibiting this practice.

My company requires all pilots to position in uniform at all times. And we normally only position with other airlines since we fly charter, and often needs to position to meet the aircraft somewhere to replace the operating flight crew.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

@CarloFelicione  I'd disagree; his answer is, it's legal & he does so routinely.  Looks like a valid (and accurate) answer to me. JMHO.

Yes it is. I had a flight attendant sit down next to me on the return flight leg of her trip. She was off duty & if I remember correctly, was in uniform. The other flight attendants recognized her as their co-worker. They treated her like gold on the plane, even though she didn't have to work that shift / flight leg. She wasn't part of the active flight crew on that plane, but she was allowed to board the plane in uniform.
The movie Catch Me If You Can details how a civilian who was not trained to fly planes, dressed up as a pilot in order to fly for free. That was a long time ago, when the aviation industry was still developing it's rules & regulations. Is it advisable to do that? No, because the man in the movie went to jail. You can check out the movie here: https://www.imdb.com/title/tt0264464/?ref_=nv_sr_1

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is there a downwash equation?
I need to find an equation for the following problem: Suppose a full-size glider passes 10 feet over my head at high speed. No doubt I will feel the downward air pressure (downwash) caused by the angle of attack of the wings. But if the same glider flies over me at a distance of one mile, I will not feel any effect at all.
So obviously there is a diminishing strength of the downward air pressure that eventually lessens to a point where I cannot physically detect it. Therefore, is there an equation that, with all relevant criteria being inputted, such as air temp, air density, wing speed, wing characteristics, wing angle of attack, etc., can allow me to at least approximate how far I must be below a passing wing to be unable to detect its downward pressure wave?

It seems that you are confusing ground effect and wake.  An airplane wake can move a long distance and still be felt, whether near the ground or not.  Ground effect relates to how the flow around the aircraft interacts with the ground.

What you probably are looking for is the Biot-Savart law.
This is originally describing the magnetic field generated by an electric current; however, it was soon found that it equally well describes the induced speeds caused by a vortex. The vortex is the bound vortex of the wing plus the free vortices trailing behind it. Integration over their length will give the induced speed at a given point. This speed is proportional to the inverse of the distance to the vortex squared. The vortex strength depends on air speed and span loading; the proportionality being with the inverse of flight speed, aircraft mass and the inverse of wingspan squared.
There is also a dependency of downwash speed with the inverse of air density at the altitude where the glider flies, but this will be canceled out when the effect at ground level is calculated. What is left is an inverse dependency on the local air density at ground level.
Note that you will only get the induced speed, not the pressure from the Biot-Savart law. In order to calculate the pressure increase, you need to add the equation for the pressure rise of the downwash interfering with the ground. This pressure is proportional to the downwash speed squared and air density, so now all density effects are canceled out and you will be left with those factors for the pressure rise from an aircraft flying overhead:

the square of the induced downwash speed

which depends in turn on:

the inverse of height squared
aircraft mass
the inverse of wingspan squared
the inverse of flight speed

Note also that this does not consider viscous effects.
In order to know the limit of detectability, you also need to know the sensitivity of your measuring equipment.

There definitely is a commonly-accepted rule of thumb:
When a wing is traveling at or less than half the wing's length above a runway or other flat srface, it will experience ground effects.
Feeling a downdraft is one thing.  But it is another thing for that pressure wave to interact with the ground closely enough to affect the airplane.
Anyway, if it was a small plane you would be more worried about getting hit by the wheels if it were in ground effect..

I really think your answer is flawed in what ground-effect is. It is not a "cushion of air bouncing up" or collecting under the wing, it is a reduction in induced drag.

@ronbeyer, that reduction in induced drag occurs because the wing in ground effect flies at a reduced angle of attack compared to the same wing not in ground effect and at the same airspeed. this trigonometrically reduces the size of the induced drag vector.

@SDsolar: Yes, downdraft is what I was looking for, not wake or ground effect, and I was informed by a physics user that a good equation to use is the inverse square law as applied to the downward air pressure, whose answer, although an approximate one, is close enough.


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How can air launched UAV's be decelerated?
I am working on disaster relief drones for a college project. I was thinking about having multiple set of drones launched from an aerial vehicle over the disaster prone area. That's when I came across a big challenge.
How would you effectively decelerate a UAV that is air launched from an aircraft which is flying at a much greater speed than the maximum allowable speed of the UAV?
The stall speed of the aircraft is much larger than the maximum speed of the UAV (assuming, it has to be a very low cost UAV and hence the idea of having a parachute for deceleration is not very effective).
I would be glad if anyone could suggest me simple and robust ways to do achieve this.

What kind of UAV are we talking about? A multicopter or a fixed wing UAV. Are you afraid the UAV could take structural damage because of the speed or that the UAV might be uncontrollable at this kind of speeds?

Use a helicopter.

What kind of drones are you thinking about? Commercially available off-the-shelf types, or custom self-built?

You should specify what you mean by "much greater speed than the max. speed of the UAV". This can mean greater than the max self-attainable speed of the UAS, which is not necessarily an issue, or greater than the $V_{NE}$ of the UAS, which would risk causing structural damage, and has no easy solution besides slowing down the mothership for this mission phase.

Thanks for your reply! I hope the following data would suffice, Drone is a deployable fixed-wing UAV (Built in-house). Its maximum allowable speed is about 0.15 Mach and the stall speed of the carrier aircraft is about 0.25 Mach. I'm open to any ideas or suggestions.

"the stall speed of the carrier aircraft is about 0.25 Mach" in cruise configuration, I assume? I mean, it has to take off somehow, unless that too is launched from another mothership.

......the same way you decelerate any other kind of aircraft.  Once released from the Mothership at idle power, it slows down until reaching an appropriate cruise speed thence power is increased to maintain speed.
If the release speed of too high for the UAV, one might use some kind of a frangible capsule equipped with a drag chute or other high drag device of some sort to enclose the drone during launch, thence the capsule fractures once the speed is slow enough, releasing the drone to begin flight under its own power.

Based on the feedback I have obtained from this forum it seems to me that a parachute system for such an application is inevitable. Is there simply no other possible mechanical method to slow it down?

What's wrong with a chute @CuthillMckee? They are inexpensive, reliable and easy to deploy using a static line system. Relief supply chute material tends to get repurposed by those on the ground into all sorts of life saving purposes like shelters, clothing and bags.

@CuthillMckee airbrakes are a thing, but they are heavier and more complex, so for a one-time-per sortie application, a parachute makes more sense.

Yeah basically a parachute or some other kind of detachable or retractable high drag kit which can slow the drone down to operating speeds prior to detaching or retracting. It need not take the form of a chute per se, but a chute is a simple and reliable option.

I acknowledge your suggestion for using parachutes. But I had the idea of utilizing the propeller (Propeller used for propelling the UAV) and let it windMill to generate drag and decelerate the platform. I am aware of some of the major drawbacks of this idea but I'm trying to soak in as many suggestions as possible to take a very informed decision.

@CuthillMckee Using a prop as a turbine sounds like an interesting topic for a PhD paper, but I am not sure I'd enjoy tackling it in the industry. How will the gearbox take it? If this is an electric motor, how will it behave when forced to dissipate a lot of mechanical energy? What about the prop pitch controls, if present, or how bad will the airfoil be for inverse flow, if it is fixed-pitch? How stable will the airframe be with the prop turned into a turbine?

You can just let drag take care of it.
Granted this means that you need to have the drone sturdy enough to handle that headwind.
Too high speed is only a real problem when you go to maneuver the drone. Straight and level flight and gently adjustments is less of a problem.
The next option is to launch upwards in a climb and wait until the drone has bled enough speed that it can safely maneuver it's way down.
A set of speedbrakes in the tail can help keep the drone stable until the speed drops enough for normal flight.

Being dropped includes going through the wake of the dropping aircraft. Gust loads and flutter will be unavoidable - there is no option to just fly straight and level when going through the wake.

One way might be to design the UAV to be deployed with flight control surfaces folded, use a parachute to decelerate, unfold the flight control surfaces at a safe speed and release the parachute.
Another would be to deploy the UAV inside some sort of canister that only falls away at a safe airspeed, letting the drone then fly away.

Thankyou for the reply. But letting the cannister freely fall would increase its speed due to the Earths Gravity pull right? It would end up accelerating the speeds probably even higher.

A properly designed canister will slow down aerodynamically. You can also install a small drogue chute that only deploys a larger chute at the correct airspeed. Parachute deployment can also be controlled with deployment rings that prevent the chute from fully opening at too high a speed.

Thank you I'll keep that in mind. So a properly designed cannister might also decelerate the UAV!

A canister that does not have its own propulsion will have no choice but to decelerate once deployed from your "aerial vehicle". What you have to consider is how to design the canister so it decelerates and allows safe deployment before it makes a hole in the ground. :)


text:
Why don't commercial passenger safety videos ever explain policy instead of merely dictating it?
British Airways announced and published their new in-flight passenger safety video:

I'd like to draw your attention to two pieces of instruction given in the video, but also stated in every passenger safety video I've ever seen on every airline I've flown:

Never inflate the life-vest inside the aircraft.
Always put on your own air-supply mask before assisting others.

These instructions are always given in a dogmatic or dictatorial, way - the airlines expect passengers to comply with these instructions despite never giving any the reason for these policies - and I'm concerned certain passengers would disregard the instructions because they think they know better (and I included myself in that category until I found out why these policies exist).
I understand the reasons, respectively, are:

While being inflated makes it harder to move around the plane when your torso just doubled in volume - more crucially: if the plane does take-on water it means you'll be trapped inside the plane because you won't be able to submerge yourself to access an underwater door or exit.
Useful consciousness at altitude is measured in seconds, not minutes, and you will likely be unconscious and suffering hypoxia if you get distracted - it will take you a good 10-15 seconds to put your own mask on, let alone someone else's. It's entirely possible you'll die or suffer serious brain damage in your selfless attempt to assist your child.

Before I was aware of these facts I simply assumed the reasons were more mundane: inflating your life-jacket would simply make things slightly harder for yourself, and you should don your own mask first simply because it's easier - and I thought that inflating my own jacket inside the plane would actually save me time, and that I believed I was perfectly capable of assisting family members with their masks before my own if I held my breath. While laughable when informed, I genuinely believed these facts about myself until fairly recently. It would not surprise me if many passengers today feel the same way about themselves.
I understand the the real reasons for the policies make for grim reading and putting those kinds of visceral descriptions in the videos would spook passengers - but I assumed that because the videos never give any dire warnings, and are always said with a cheerful tone of voice, that the instructions weren't all that important, so it would somehow be tolerable if I did my own thing and disregarded the rules because I thought I was acting in my own best interest.
So why don't they at least add some transparency and explanation, for example they could say:

Never inflate your life-jacket inside the aircraft so you can fit through smaller openings and to avoid being trapped if the aircraft takes on water.
Always put on your own air-supply mask first before assisting others because seconds matter in low-oxygen situations.

While the sample explanations I gave don't go into too much detail, they still make it clear to passengers that the consequences of noncompliance are serious, if not fatal - and could still be read with a cheery tone of vocie :)
I compare this to the problem with "Danger: Confined Space" warning signs: the layperson thinks a confined space is dangerous because they might casually bang their head on a low ceiling - no thought is paid towards the common, very real risks and dangers of dangerous gasses pooling in a narrow shaft - yet it isn't too much trouble to add a short message to the sign saying "Because of trapped carbon-monoxide and other gasses", for example. It's odd because we already have other explanatory danger signs like "High voltage" (don't touch it without gloves), "Moving parts" (don't touch it, period) and "Falling objects" (so wear a hard-hat) - so why not airline safety instructions?

"Why don't commercial passenger safety videos ever explain policy instead of dictating it?". Nothing prevents doing that, so why did BA make this choice is a matter of opinion unless one works with BA, and reasons are likely remote from aviation.

IIRC some airlines do explain the reasons, Virgin Atlantic is one of them. They've made their safety announcement entertaining, which gets a lot more people watching it, one of the things I like about their approach.

In some ways, this is almost a question for the User Experience Stack Exchange. My not-answer (because I don't have sources to back me up at this time) is that to be as non-confusing as possible to the broadest audience, and to have the best chance of being remembered in an emergency, direct instruction is given rather than explanation. Yes, for some people, an explanation will stick better than an order, but that's not true of everyone. I also think that your comparison to safety signs is apt. For clarity and broadest compliance, they keep it short and simple. +1 from me for the question.

@Dranon: "to have the best chance of being remembered in an emergency, direct instruction is given rather than explanation" This is possible, but arguable. I've seen people that don't do recommended things because they just don't understand the purpose, and in addition don't trust the person who requests such action (they think their life is better protected by their own reasoning than by the reasoning of a commercial entity -- and this that can be true from time to time). However if better informed of the consequences, they might just do as suggested.

At least they explained why you had to take off your high heels

Why would they explain why?  It would just confuse people: it's a rule and you have to do it.  When you see a speed limit sign, do you expect an explanation?

actually in that one they DO add a quick rationale for removing high-heels.

It's entirely possible you'll die or suffer serious brain damage in your selfless attempt to assist your child. Quite unlikely. You will just pass out.

@Dai - Slightly off topic, but you don't actually seem to understand why you should put your own mask on first! The key point is twofold: time until someone without a mask passes out - relatively short; time until someone without a mask gets permanent injury - relatively long. If you put the mask on an incapable person: they'll be fine but you'll then pass out, and the incapable person can't put your mask on you - you then die. If you put your mask on first the incapable person will pass out, but you are then able to put their mask on them before permanent injury occurs.

I've always thought it was because they don't want to draw attention to the dangers of flying. If you tell somebody "make sure to put your mask on first, or else you and your child might die" then what they'll hear is "YOU AND YOUR CHILD MIGHT DIE." But that's just my speculation, and SE is supposed to be about more than speculation, so I'm not posting that as an answer without a source to cite.

Wait... is it possible to suffer hypoxia while holding your breath at the same rate as you would while breathing thinner air?  Or would it in fact buy you a (small) amount of time?

Actually, it's not just that you might die as a result of helping your child, your child can die as a result too ...

On my recent trip, I think they explained why not to inflate the life-jacket while still on the plane - with Ryan Air of all airlines. (I don't remember exactly, but it was something simple like "So you won't block the exits").

@Syzygy "...with your body".

If they explain the reasoning, they invite discussion, counter argument, thought, and alternatives.  None of which will actually change the rule.  Someone may believe "I'm a strong swimmer, and athletic:  those life-vest rules don't apply to me", but the rule still applies.  They don't explain the reasons, because the reasons don't matter.

IMO everyone must see Japan Airlines / ANA's safety video. They have explained every aspect of non-compliance through images.

@HarikrishnanM Got a link?

https://www.youtube.com/watch?v=3BDIGt8MOD8

No taking pics during evacuation, no heels on the slide, no baggage taking,  etc etc... might be very little, but enough I guess?

Japan is very particular about aviation safety, and this is an example. They explain the details of the evacuation and how it should look like too.

It's true, this is a UX question.
DROP THE WEAPON! The power of clear, directive communication is to get you to act fast.
Whereas if you try to explain the thing, people's minds start to turn, and for lack of anything better to do, they start to argue. And it turns into this.
The other thing is that for anyone who is cognitive-impaired, or language-impaired, or just in a high-workload environment... when you add words, you add confusion. Hence brevity codes.

You are not asking anyone to do anything immediately. Your target is to get people remember what to do. I would actually think, that explaining the reasons would help understand, remember and act correctly when the situation comes.
At lest for me, after seeing this video I now understand and probably remember far better in an emergency why to put my mask on before helping my children: https://www.youtube.com/watch?v=XcvkjfG4A_M

The video is disturbing because it rings so true...airplanes use magic :)))

There is no time. You need to convey the information in as little time as possible or people (those few that watch at all) lose interest and start doing other things.
Reasoning tends to invite arguments, there's always someone who thinks he knows better. You don't want that.
Simplicity. Make things as easy to understand as possible using simple words. That way less intelligent people and people who have trouble with your language are more likely to understand what's expected of them.

There might be other reasons, but those are the ones that come to mind.

I tend to disagree. Especially for life jackets. Ryanair in particular adds as this will impede your exit. How long does that take to say? 3-4 seconds maximum? But now you know: inflate jacket before exit -> drowning. Let's not forget that according to Wikipedia in Ethiopian 961 Many passengers died because they inflated their life jackets in the cabin. I guess they were also told not to inflate them...

@SteliosAdamantidis many people don't listen to the announcements and so will not know what to do. Others panic or are stubborn and do things even when told not to. Most airlines say something like 'this will impede rapid evacuation' but don't go into details like mentioning the results of studies done that show that, or section and paragraph of the law that requires the procedure, which is what "explaining policy" would entail.

Also: Aviation regulations do not state how an airline should deliver the briefing, only that ‘The operator of an aircraft shall ensure that all passengers are orally briefed before each take-off’. (excerpt from Wikipedia). So why waste more time doing it? IMHO those things cab be teached in school put togheter with another unlikely scenarios like saving someone from a hearth attack

Generally yes, @Kik, but I believe the situation in an aircraft is a bit special. It is obvious to everyone that what they are doing is there to save your life. At some point, it should be possible to just accept that they probably know what they are doing. Would you want any warning/danger sign (on trains, in factories, whatever) to explain in detail why it is important to do X, Y or Z here?

In a real emergency, say in a car accident, if I need some bystander to call the police or ambulance, I would address him firmyl and say very clearly, "You! Call an ambulance now! Take your phone and dial XYZ. Go!" to increase the chance to get through their excitement or whatever state they are in. I would never get the idea to start explaining why. The same applies here. Obviously the emergency is not there yet, but if one should ever happen, people might just remember the clear command instead of some convoluted explanation.

As a system administrator with 20 years experience communicating with users, the exact same truths apply to IT policies and procedures. The only time people will care about the reasons is to argue with them, and the only people who will even understand the reasons already know them.

It's pretty much safety theatre, anyway. Crashes are unusual. Survivable crashes into water are a once in a generation thing. The taxi ride to the airport is 19 times more dangerous than a typical flight, but you don't get a safety briefing (or a helmet).

@SteliosAdamantidis Sorry, but Ryanair's "this will impede your exit" doesn't explain anything. It's just another dogmatic statement, and it might make somebody ask themselves "why would it do that" and not listen the next 30 seconds of the briefing because they were still figuring that out.

@AnoE That's exactly right. I still remember my exact safety briefing on day one at work, about what to do in the event of an imminent hazardous chemical release: "If you are brave, you will shout 'Acetylene leak! "Evacuate! Everybody out!' and then run like **** for the exit. But if you are wise, you will just run like **** while you are still alive". If I had been given a 5 minute chemistry lecture, I would have forgotten the details 40 years ago, if I ever remembered them at all.

@SteliosAdamantidis Just adding that (not very explanatory) second sentence almost doubles the message. Explaining the reasoning for the rule properly takes even more time.

@ToddWilcox as a software engineer with over 20 years of experience I know full well that many sysadmins make rules for the sake of giving themselves power and allowing themselves to appropriate resources that shouldn't belong to them without repercussions. Take the network admin I had to deal with who decreed that streaming music was not allowed because it was too heavy on the network, and on investigation was found to hog the network with the servers he'd running 24/7 downloading pirated movies he burned on DVDs (using company resources) and sold on eBay...

@jwenting exactly this. In my last department, when the draconian sysadmin left, everyone sighed with relief, because things suddenly started working again. I'd counter "the only time people care about the reasons is to argue about them" is because the reasons are usually daft and more to do with control than reasonableness or utility. (whether the rules could be re-written by this point is another issue of course). And "people who will understand these reasons already know them (and by implication keep quiet)", translates as "they already know how to bypass them and are doing just that".

@jwenting That particular example doesn't seem to be a good one. Prohibiting streaming is actually a good idea, and whether a particular admin is a lawbreaking hypocrite or not does not change what the best practices are. I actually worked right next to a guy who was on the IT team and I had to be the one to tell our boss that the firewall traffic reporting clearly showed he was downloading HBO shows from bit torrent to his work computer. And yes we still prohibited streaming on that network.

@ToddWilcox that was just one example, same guy was using keyloggers to steal the passwords from our workstations and turning them into p2p filesharing bots, then complain about us using too much network traffic... And that's far from the only one acting irate. What about the sysadmin group (of about 5 people) assigning themselves 99 out of the 100MBit pipe available to a 1500 person company? Or the sysadmins demanding contractors bring personal laptops ('bring your own device" policy) but then refusing to allow any hardware not supplied by them on the corporate network "for security reasons",

I can understand such a policy, but not in combination with a policy demanding people bring their own devices, which they know they won't allow to be used...

I would downvote this answer if I could. The example video in question contains lot of redundant stuff.

@jwenting I don't understand your point. Is it that one example of hypocrisy undermines any assertion of what is a best practice? So if a flight attendant ever has inflated their life vest before exiting the plane, then people should no longer be told to wait until exiting the plane to inflate their life vest? You seem to be validating this answer: "Reasons invite argument". Also, software engineers are not typical users the same way airline maintenance crew are not typical passengers. We don't make policies and guidelines for software engineers, we make them for CEOs.

@ToddWilcox just countering your statement that all sysadmins are by definition always right and anyone disagreeing with them by definition always wrong.

@jwenting You're also providing an example that providing reasons tends to invite arguments. :)

@ToddWilcox true, and potentially valid arguments :) Which in an office environment is good, an airliner cabin isn't the place for them.

In a word: fear
"Increased perception of risk" would be a longer way to say the same.
Consider that the more you expand on risks and dangers, the more dangerous and risky the activity seems.
As a parallel example - people who want to ride a bicycle after an absence express an increased fear after they're told to wear a helmet, and wear a high-vis vest, and carry lights at night, and wear gloves, etc.
Spending a heap of time explaining about uncommon scenarios will increase the perception, and hence the perceived risk.
Final example from a meme:  "With the amount of stop drop and roll training we did I thought I would be on fire a lot more as an adult."  Increased perception again.

Because they don't have to.
Air carriers are not required to explain why, so they may choose not to. They are required to instruct, and there is a long list of instructions that are required. Air carriers follow what is required of them, at a minimum.
As a crewmember tasked with briefing our air carrier's passengers, I have a long list of items that I have to cover. I cover the items that I have to cover, and do so as succinctly as I can. I may choose not to do too much explaining or go into too much detail on why they may need to know about the location of exits, survival gear, and fire extinguishers. Passengers could become anxious if you explain the consequences of not wearing a seat belt, not promptly initiating use of the O2 mask, etc. They are already staring at you wondering why you are saying all this and are probably wondering how much longer till they can return to reading the newspaper. Explaining time of useful consciousness could be disconcerting. I would rather tell them simply to put on the mask; after all, I will take care of getting them safely down to a reasonable pressure altitude anyway.
Now, if I was taking a close friend flying, I might go into more detail about what we are doing and why. But for the traveling public, I choose not to.

"Air carriers are not required to explain why, so they don't", When not required to do something, airlines don't do it, it this your opinion or do you have something to support this statement? Is this valid for any domain, price increase, safety, crew training... all at the minimum required by law?

@mins I would not espouse the notion that an air carrier will categorically avoid any action not expressly required of it. In this answer I offer my experience and reasoning as to why air carriers don't go into more detail. There are likely other reasons. The air carrier that I work for actually does include additional details in our preflight briefings beyond what are expressly required (location of briefing cards, use of personal electronic devices).

@mins Of course it's valid. Welcome to the real world. If a pilot can get a type certificate on an airlines new aircraft type with 5 hours ground training and a check ride, no business that aims to make a profit will give them 20 hours ground training, 10 hours flight training, and two check rides a month apart just to make sure they haven't forgotten anything during that month! But balance that against the fact that civil aviation is one of the most heavily regulated activities on earth today. That's why it's such a safe method of transport.

Edwards: Why the big secret? People are smart. They can handle it.
Kay: A person is smart. People are dumb, panicky dangerous animals and you know it. Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know tomorrow.

Yes, I just quoted Men in Black as an authoritative source, but never mind that.  The logic is surprisingly sound.
The airlines don't care if you remember not to inflate your life vest before leaving the cabin when everything is just fine.  They care that you remember these things during a panicky situation where life vest inflation is a real concern.  When this happens, your mind doesn't work.  If you were dependent on your mind working, you are probably dead.
We have a lot of experience with how people react in these situations.  It turns out its more effective to command people to do things rather than explain to them why they should do it.  If they memorize the reason, they may try to think their way through the event to see whether the reason qualifies right now.  Without a working mind, that's a big issue.  In emergency situations, it's more important to do than think.
Now some people can overcome this.  You may be someone who can respond better by knowing why oxygen is flowing to the mask, even though the bag may not inflate.  But they have found that, in an emergency, the people they need to cater to are better supported by giving orders, not giving reasons.
And, if you're someone like me, you may choose to look up why the bag may not inflate.  That's your prerogative.  Their job is to ensure safety.
And if you're curious:

 The "dixie cup" masks have a constant flow rate of oxygen into them.  This is much cheaper than other forms of masks, but human breathing is not constant.  The bag acts as a buffer.  It also ensures that you rebreathe the air.  If you start hyperventilating, which would waste oxygen, the bag will inflate more, capturing more of your wasted oxygen for the next breath.  If you breath smoothly, you may not see the bag move at all.  (Good luck breathing smoothly!)

"In emergency situations, it's more important to do than think." It's a nice phrase, but probably wrong. You can find as many proofs of this statement than counter-examples. Hudson ditching has been done after thinking about all possibilities.

@mins Actually, I would it is the definition of an emergency situation.  An emergency is a situation where you must act.  The best stay cool under pressure and succeed at thinking while acting, but the acting had to happen.  Consider that, if someone takes charge, they can think, while everyone else just acts.  That typically works in an emergency.  Contrast that with the situation where everyone thinks and nobody acts.

@mins Actually that's a classic case of pre-rehearsing scenarios so that you know the options, rapid evaluation of options, and simply playing the scenarios for those options.  The first fallback is always to land back at the runway; when that wasn't possible, Sullenberger already knew about Teterboro; and when that wasn't possible he had enough experience to have ditching as a final strategy.  He didn't have to think deeply about it, just look at the situation and react.  The radio traffic with the tower wasn't a discussion, it was simple statements of "this is what I'm doing".

Those who act first and try to understand after are bound to reach deadlocks more than the others as soon as the situation is not trivial. Much of the content of this page is plausibility dressed up in truth... step back and read again. I don't challenge the ultimate choice of BA, but none of the answers contains the beginning of a credible explanation.

Actually very few people believed the earth was flat.

And we are the centre of the universe  because centre is pretty meaningless.

@mins actually I recently underwent training in how to manage behavioral outbursts in people with cognitive and psychiatric disabilities. The first rule was to always STOP and THINK before acting during a crisis, as the instinctual response is normally to physically restrain the person acting out, which is almost always the wrong response, as it tends to cause behavior escalation and, if done improperly, likely constitutes reportable Abuse of a Vulnerable Person (i.e. your career is over and you are probably going to jail).

Ryanair explicitly says "in the unlikely event of landing on water [...] do not inflate your jacket before leaving the plane, as doing so will impede your exit."
Since you refer to BA, I think it is only a matter of BA's choices.

"Impede your exit" doesn't imply "you'll be trapped in the plane as it sinks and you'll drown" though - I'm concerned it trivializes the danger.

@Dai It doesn't? If you're impeded from exiting an aircraft that has landed in water, what do you think is going to happen?

@reirab Well, "impede" doesn't sound like "prevent"

@HagenvonEitzen Impede means to "delay or prevent by means of obstruction." As a native speaker of American English, it seems like perfectly appropriate word choice here to me, but perhaps it's not as obvious to non-native speakers? If so, that would indeed be a good argument for changing the word choice, since, obviously, lots of passengers are not native speakers.

@HagenvonEitzen: Well, if you're an extremely strong swimmer, it's possible you might be able to exit the aircraft even with an inflated vest, in which case it would only "impede your exit"...

The US regulation requiring airline cabin safety briefings is here:
https://www.law.cornell.edu/cfr/text/14/121.571
...and ends with "(c) The certificate holder shall describe in its manual the procedure to be followed in the briefing required by paragraph (a) of this section."  So, the requirement to brief passengers on specific items is a legal requirement, but the FAA allows each carrier to create its own procedures for doing so.  The procedures in the "manual" referred to also require FAA approval, so individual airline staff aren't given a ton of latitude on what to put into or leave out of a safety briefing.  There may also be time constraints involved, depending on how long the cabin crew has to get the briefing done, complete cabin checks (seatbelt compliance, carry-ons secured, etc) and then get sat down and belted in themselves before takeoff (all regulatory requirements.)
If you look around during the average cabin briefing, you'll note that at least half the passengers are already not paying attention, so making it longer and more detailed may not actually help much - and there are enough fearful flyers among the customers that enumerating the exact consequences of failing to comply with crew instructions isn't necessarily going to calm the masses.  Most flight attendants would be happy to answer any specific safety questions when they have time during the flight, so if you're wondering "why" things are briefed the way they are, the additional information should be available.
Loss of cabin pressure is unlikely to have any permanent effect as the pilots have their own quick-donning emergency oxygen masks and after putting those on will respond by initiating a VERY enthusiastic descent to 10,000 feet as soon as possible.  Time of useful passenger consciousness at typical cruise altitudes (mid to high 30s) is less than a minute without supplemental oxygen, but it's 30 minutes or so at 18,000 feet - and the aircraft will be there in something like one to two minutes on the way down to 10,000 where most people would be fine without masks.
To the extent practical, the cabin crew will also provide additional reinforcement of the instructions if an emergency actually occurs (e.g., "Do NOT inflate your vest until you're out of the aircraft!"), so there will be more than the initial briefing if things get ugly.

This is a minor point, but the air carrier's manual requires FAA acceptance, not approval.The FAA sometimes makes a big deal about the difference between those two words.

There are some further answers not mentioned yet.

They do not know the reason themselves. Many, many rules have a history when someone has made a discovery and shared his knowledge about it. People have a tendency to forget or misinterpret the original reason and only stick to rules because, you know, once you follow it, you are safe. This is a problem because once a rule becomes obsolete for the reason that it was errornous or does not apply anymore, people stick to the original rules.
In victorian times the man should go before the woman upstairs so he cannot see her legs. Later the rule was changed because women were wearing boots with heels, long skirts and sport was unwomanly, so they could easily feint or trip and the man is at least able to catch her. Now in modern times the rule is actually obsolete, but still followed.
So good luck trying to find someone in the airline who actually knows that people were drowing with their life jackets or that people lose their mind and rapidly pass out without oxygen.
People are different. You see that already in school. The more right-leaning pupils are believing that rules must be obeyed and challenging them is rude. The more left leaning pupils are suspecting that the rules are there to impede their freedom and are prone to do exactly the opposite. Other pupils are not taking attention, take no interest in their surroundings and don't listen. (This applies equally to adults. My ex-girlfriend was driving the urban railway to home when a tornado was hitting southern Hamburg, the railway stopped and passed 30 m apart from her over the bridge and knocked over the river trees. As incredible as it sounds, she insisted that nobody else seems to even notice it).
So as many people here already mentioned it, it may make sense to address the instructions in a form that most people are aware of it (and CYA for the airline...You did not listen to the instructions?).
Less attack surface. "Put on your mask immediately when it falls down". (Passenger did not do it and dies). "The person did not comply with our instructions and we are therefore not responsible".
"Please put on your oxygen mask because it will take only 15 seconds to lose consciousness". Shyster: "My client has witnesses who can certify that he carefully used a stopwatch to put on the mask after 12 seconds, but he lost consciousness before the time. So the airline is responsible because my client trusted them with his life for the correctness of the claim". Blah. Blah. Blah.
People are prejudiced, biased or even plain dumb and tend to misinterpret explanations. Every author of a controversial topic could tell you long tales of what people assumedly found in their books which they have never written, even the contrary.  Heck, if you follow discussions here, you are often surprised what people interpret in your comments (Hm..what???).
When Feyman was working on the atomic bomb, he made an interesting observation about the safes: Once they are open, due to a design flaw you could easily take the first and second combination component inconspicously in seconds. He demonstrated it before a colonel who was flabbergasted, explained it to him in detail and said: "Please instruct people that they do not leave their safes and cabinets open". What did the colonel? He instructed:"Everyone who was in contact with Feynman should change their combination".

"The more right-leaning pupils are believing that rules must be obeyed and challenging them is rude. The more left leaning pupils are suspecting that the rules are there to impede their freedom and are prone to do exactly the opposite." What? This is not only incorrect and overly broad, but just generally inflammatory.

It is possible the technical staff at the airline disagree with the safety policy that is required by law. So, to announce the policy but attempt to minimize compliance, they simply state the policy with no explanation.

@ThorstenS. I would classify that as "libertarian" rather than left-wing or anarchical.

@Dai AFAIK in the UK it fits exactly, but in the US it is tightly associated with unrestricted capitalism. "Liberal" is also a false friend, it means in the EU mostly what "Libertarians" means in the US. Also most liberals/libertarians still believe that the government should wield power over individuals to protect rights and act as judge in conflicts. Anarchists in contrast do not accept authority at all. I avoided "liberal/libertarian" because it helds different and conflicting meanings (difference e.g. between libertarian socialists and anarcho-capitalists!).

You'll hate this, but IMHO you misunderstand the purpose of the briefing/video.
It's primarily safety theatre to ensure that the "self-loading freight" (sorry,  "valued passengers") feel that they are embarking on a slightly hazardous enterprise but are in good hands due to the skills of the airline staff.
In fact, the chances of anyone being in a life-threatening incident on a civil airliner are vanishingly small. Even for airline employees, the vast majority will go through their entire career without such an incident. When incidents do occur, they are often either non-threatening or a total loss (no amount of seat-belts, oxygen masks or locked away tray tables will save you when the plane slams into a mountain at 600mph).
Commercial flights on small aircraft are a lot more hazardous than jets, but omit the safety video and often any kind of briefing beyond the pilot looking over her shoulder and checking y'all strapped in, eh?
Flying would still be safer than driving if they had no cabin crew, took all the seats out and had the passengers hang on to grab rails, just like the airport bus (which is way more likely to come to a sudden stop than an airliner).

While some (or even all) of this might be correct. It's not an answer to the question.

authors: Vladimir F Героям слава


text:
Is a 3 horsepower engine powerful enough for a 100 pound ultralight?
I'm building an ultralight and it weighs 100 pounds empty. My only engine is 3HP. Is that good enough?

The question in your title doesn't match the question in the body text, at all. And, without knowing anything about your ultralight vehicle, there is no way to say how much power it requires. Honestly, do you think it would be safe to (attempt to) go flying in something simply because somebody on the internet who's never seen your machine said, "yeah, that's enough power" for you?

Of course I would.

With the given parameters you can compute a theoric takeoff acceleration and see if you exceed take off speed in a reasonable length. For the flight, remember that glider can fly without power

Highly related: What are the least powerful airplanes that ever flew? (if not duplicate)

In general, regardless of the characteristics of your aircraft, 3 HP isn't enough power to lift the weight of the pilot safely.  Your ultralight would be effectively a power-assisted glider, unable to climb in calm air.  You'd have to launch like a hang glider (from a slope or cliff, or by one or another form of tow), though if your wing and layout are efficient it's likely you could stay up until you run out of fuel in calm air with careful, conservative flying.
Go look up some of the earliest, 1970s and 1980s vintage powered hang glider attempts.  Some tried installing engines in that range.  None kept at it, because very poor climb is an extremely unsafe condition.

It depends on the lift-to-drag ratio, and the lift versus airspeed characteristic of your wing. If you know the total weight of the plane with you in it, then you know how fast you need to fly to generate an amount of lift sufficient to exceed the dead weight of the plane- in the absence of drag. Then knowing the L/D ratio, you can solve for the drag force generated at that airspeed- which the engine must furnish- and which will subtract from the thrust generated by the engine. You then iterate the process to converge on the horsepower needed to propel the plane fast enough to lift off.


text:
What is the seaplane in this old photograph?
Can you help identify this large apparently water-borne machine?

What is the source of the photograph?

It’s easy to do a Google Image search: “The Pateras-Pescar-Guidoni seaplane taken in the Royal Arsenal of Venice in 1914” https://www-elicotterienavi-com.translate.goog/aeromobili/STORIA2.HTM?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

More info from Google image search: https://www-airwar-ru.translate.goog/enc/bww1/pescaraguidoni.html?_x_tr_sch=http&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

For those who have never used Google Image search it's on the Google Home page, on the right side of the search bar is what looks like some type of colorful plant, that's actually a microphone for voice command, to the right of that is a sort of multi-colored box with a couple of dots on it, that's supposed to be a camera, click that, on the next screen click Upload a File, from there you can select a photo that you have downloaded to your PC.  As for the icon, I guess the dot in the middle is a lens, rectangle on top is a viewfinder, dot on the lower right is - stop down metering button?

Google Image search led me to this website, which indicates it's the
Pescara-Guidoni Torpedo Seaplane


text:
Why do we see "page intentionally left blank" so often in aviation related documents?
In aviation, we see the phrase "This page is intentionally left blank" quite often in documents like POHs, AIP etc. I can't remember seeing this in any other field. Why is that the case? Why do we see it that often in aviation related documents but nowhere else (anymore)?
There is a good answer to a general question about this phrase on UX.SE but I'm wondering specifically about its usage in aviation related contexts, especially in an age where the main distribution channel for documents is digital distribution and not paper anymore.

"Intentionally left blank" is used all over the place in technical documents. Intel, government contracts, reference books

This comment intentionally left blank.

You can print out digital documents and so it matters. Computers exist for printing more papers faster ;-)

I feel like a recent edit to this question significantly changed the meaning of this question; I've edited it again to try to change the meaning back to what the original poster intended. (The edit I'm talking about changed the meaning from "Why is the phrase 'this page intentionally left blank' used?" to "Why are there blank pages?")

It's not just in aviation. The same thing can be found in official documentation from many governments and industries. It assures the reader that the page was not accidentally left blank (e.g., because of a printing error or a collation error).

Related: https://aviation.stackexchange.com/questions/62504/why-does-14-cfr-have-skipped-subparts-in-my-asa-2019-far-aim-book

I've worked as a technical writer on flight and maintenance manuals in a previous life.  It's done with technical publications where there is a kind of quasi-legal status you might say, insofar as the information published is required for operation or maintenance, is invalid for use if not up to date, and where pages are revised, numbered and dated with a List of Effective Pages (LOEP) that goes at the front, that confirms the issue date of each page.
It provides a ready confirmation that a blank page is not a misprint (even as a PDF or other electronic format), saving the reader from having to check against the LOEP to make sure there isn't a missing page every time a blank page appears at the end of a section.

Chapters conventionally have started on odd numbered pages, so if a chapter ends on an odd page a blank is inserted on the next even page. The reasons you'll see intentionally blank pages on digital documents are:

Conventions die hard in an industry as regimented as aviation
A document may be in both digital and print, there's no point in having 2 versions, and you need the versions to be identical or you could introduce problems. If one person refers to the digital and another a print version the pages need to be exactly the same

@notts90 no it’s also about the why blank pages

A lot of technical documents don't use page numbers but a section numbering scheme. This is because revisions will result in page numbering changes anyway.


text:
How did ATC procedures and airspace structure change after San Diego PSA collision?
In 1978, a Pacific Southwest Airlines aircraft collided with a Cessna 172 while approaching the San Diego airport in California. The GA aircraft was piloted by a student, and crossed the approach trajectory of the B727, killing 144 people.

Source
To my understanding, ATC was not aware of the Cessna actual location, The airliner crew didn't see the Cessna, and the Cessna was not wrong being in this airspace near a busy regional airport.
What changed after this accident to make such situation impossible?

Are you looking for changes specifically in the San Diego area, or more generally?

The Cessna was being piloted by two licensed pilots, both of which held commercial pilot certificates.

@DeltaLima: More generally.

The recommendations from the NTSB report are:

Implement a Terminal Radar Service Area (TRSA) at Lindbergh Airpot, San Diego, CA
Review procedures at all airports which are used regularly by air carrier and general aviation aircraft to determine which other areas require either a terminal control area or a terminal control radar service area and establish the appropriate one.
Use visual separation in terminal radar service areas only when a pilot requests it, except for sequencing on the final approach with radar monitoring.
Re-evaluate its policy with regard to the use of visual separation in other terminal areas.

The most direct change stemming from this accident is the establishment of a TRSA at Lindbergh on May 1, 1979. (It is now a class B area).
Also in 1979 the Lindbergh tower was equipped with some upgrades, including minimum safe altitude warning and conflict alert enhancements.
The other recommendations are fairly vague and I find no indication what, if any action was taken on them.
But as Gerry pointed out in comments, this accident put emphasis on the FAA's effort to create TCAS.  From the TCAS II V7.1 Intro booklet:

In 1978, the collision between a light aircraft and an airliner over San Diego served to increase FAA's efforts to complete development of an effective collision avoidance system.

The see-and-avoid system is still very much in use. Visual approaches are still used. There really were no immediate systemic changes made in response to the accident. Mostly it was just airspace configuration around KSAN and enhancements to the radar system there. But, like every accident, the NTSB and FAA learned from it and took steps to improve safety. The FAA releases pamphlets appropriately titled, "Lessons Learned," about what can be taken from different types of accidents.

It also put emphasis on the FAA's effort to create TCAS.  From the TCAS II V7.1 Intro booklet:  In 1978, the collision between a light aircraft and an airliner over San Diego served to increase FAA's efforts to complete development of an effective collision avoidance system.

@Gerry Good find. Thanks. Edited your quote in to improve the answer

In addition to TomMcW@'s answer, the FAA also installed ILS systems at both KMYF and KCRQ and implemented Class B airspace around KSAN.
The Cessna wasn't just allowed near busy airspace, they were specifically doing ILS training at the airfield executing a missed approach under VFR.
Adding ILS systems at GA airports didn't directly change ATC procedures or airspace definitions, but it result in fewer GA aircraft needing to use KSAN for training.
Furthermore, PSA 182 did report traffic in sight but they continued their landing procedure anyway once they lost visuals.  The fact that ATC didn't know precisely where the Cessna was and recognition that accidents happen even when aircraft report that they are maintaining visual separation also directly lead to Class B airspace being implemented at KSAN:

As a result of the crash, the NTSB recommended the immediate implementation of a Terminal Radar Service Area around Lindbergh Field to provide for the separation of aircraft, as well as an immediate review of control procedures for all busy terminal areas. This initial rule did not include small, general-aviation aircraft. Therefore, on May 15, 1980, the Federal Aviation Administration, implemented what is called Class B airspace to provide for the separation of all aircraft operating in the area. Additionally, all aircraft, regardless of size, are required to operate under "positive radar control", a rule that allows only radar control from the ground for all aircraft operating in the airport's airspace.

(Wikipedia: Pacific Southwest Airlines Flight 182)

That quote from Wikipedia is misleading to the point of being incorrect. VFR-IFR sep in Class B is only one of: 1.5NM radar sep, 500' vertically, or visual separation. (And that's only if the other aircraft weighs more than 19000 pounds; otherwise the radar separation drops from 1.5NM to target resolution.)


text:
How is the search-and-rescue system for civil aviation over waters agreed and funded?
Several accidents in the recent years have revealed how important --and intuitively expensive-- SAR operations over seas can be.

Need for daily government agencies / military flights in the first weeks after the accidents, need for underwater search.
SAR managed by other countries than the registration country of the aircraft.

Crewmembers of an Indonesian Air Force NAS 332 Super Puma helicopter over the Java Sea, Indonesia, 2015.Source
Without entering into too much details:

How is SAR for civil aircraft agreed globally?
Is that under ICAO umbrella?
Is it shared with maritime SAR or other organizations?
How is SAR responsibility shared between countries worldwide?Is a map with responsibility boundaries available?
How is SAR funded?
Are operators (their insurances) liable for costs?
Is it a service provided for free, if so to which extent?
How is the decision to stop active search taken?

Search and Rescue is under the ICAO umbrella, but also with the IMO.
Annex 12 to the Convention on International Civil Aviation gives the international standards and recommended practices (SARPS) on Search and Rescue.
The annex is supplemented by the International Aeronautical and Maritime Search and Rescue (IAMSAR) Manual, which is published by the International Maritime Organisation (IMO)
Together the documents describe:

The organisational structure of the worldwide SAR system concept,
including cooperation between states. (ICAO Annex 12, chapter 2, 3 &
4, IAMSAR volume 1)
The operating procedures for organising and coordinating a SAR
mission (ICAO Annex 12, chapter 5, IAMSAR volume 2)
The operating procedures for those carrying participating in the SAR
mission (Appendix to ICAO Annex 12,  IAMSAR volume 3/ ICAO doc. 9731)

The details of implementing the SARPS are left to the states themselves.


text:
Why the dynamic pressure is not mentioned in the explanation of lift by Bernoulli's principle?
Usual "explanation" of lift using Bernoulli principle
This theory is:

Air over the wing is accelerated, its dynamic pressure increases with velocity.
The static pressure over the wing decreases (due to Bernoulli principle, see below).
The opposite occurs below the wing, static pressure increases.
This results in an imbalance in static pressure between sides, and explains lift origin.

Source

Bernoulli's principle
Bernoulli's principle explains "an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy":
$$p + \frac{1}{2}\rho V^2 + \rho gh = k$$
where $k$ is a constant. In the case of a wing, $\rho gh$ is so small it can be omitted, the principle can be stated:
$$p_0 = p + \frac{1}{2}\rho V^2$$
where $p_0$ is a constant known as total pressure, the first term is the static pressure, and the second one the dynamic pressure.
The explanation of lift ignores the imbalance of dynamic pressure
The previous theory ignores a similar, but opposite, imbalance of the dynamic pressure (as the total pressure is constant, the arithmetic sum of the two imbalances is also constant):

Why is the force created by static pressure put ahead in the explanation of lift, and the force created by dynamic pressure is not mentioned? If this force is ignored because not significant, why and how much is this force smaller?

In short, "dynamic pressure is not pressure at all" (A Princeton paper on dynamic pressure and Bernoulli's theorem).  "Dynamic pressure" is a term that designates the decrease in pressure, as a consequence of the fluid's / medium's flow velocity.
I recommend that you also examine what NASA has to say:

Integrating this differential equation:
ps + .5 * r * u^2 = constant = pt
This equation looks exactly like the incompressible form of Bernoulli's equation. Each term in this equation has the dimensions of a pressure (force/area); ps is the static pressure, the constant pt is called the total pressure, and
.5 * r * u^2
is called the dynamic pressure because it is a pressure term
  associated with the velocity u of the flow. Dynamic pressure is often
  assigned the letter q in aerodynamics:
q = .5 * r * u^2

The dynamic pressure is not a pressure as normally perceived.
From the wikipedia article:

Dynamic pressure $q$ is the kinetic energy per unit volume of a fluid particle.

$$q = \frac{1}{2} \rho u^2$$
I.e., it is the increase in static pressure that you would have if you would slow down the fluid to a complete halt, but until that moment it is kinetic energy that is not "pressing" on anything, the same way the kinetic energy of a car is not pressing on the road it is travelling (but would press on a wall that would try to suddenly stop said car).
It is well visualized in the image included in the wikipedia article as well

The flow on the left is travelling faster, and has increased dynamic pressure, but as the static component decreased, it is pressing less against the walls of the glass tube, leading to a pressure differential.

Why does dynamic pressure reduce static pressure?

@Koyovis that's not how it works. as stated in the question, total energy is constant, hence if one component increases, the other decreases.

Adding my own answer to summarize some elements.
Pressure is not always pressure
This wording dynamic pressure is indeed very confusing, as this is not a pressure which can be measured with a barometer or that kind of instrument. Similarly total pressure itself is not a pressure either. In Bernoulli's principle which is summarized in...
$\text {Total pressure} = \text {static pressure} + \text {dynamic pressure} = \text {constant}$
... the only pressure is the static pressure. Instead total pressure and dynamic pressure are energy. So to answer the question: Lift is a matter of forces exerted on the wing, and the only force involved is the one from the pressure (i.e. the static pressure).
Stagnation point
An interesting way to look at this equation is to start from the situation where the flow is interrupted by a body. The flow must take another path, for example:

The flow is deflected in all directions around a central point. At this point air is blocked and accumulates until some point, preventing more air to come from the direction of the flow. This point where air is stagnant is the stagnation point. As air is not moving here there is no "dynamic pressure". Per Bernoulli's principle the total energy is converted into pressure, and this point is the location where pressure, known as stagnation pressure, is maximal.
At any other point, energy is split between flow (static) pressure and kinetic energy (dynamic pressure).
Pitot
On the other hand, if the wall is moving and air is static, at the stagnation point one side of the wall is subject to total pressure, and the other to static pressure. The difference is therefore the dynamic pressure which is known to be proportional to the velocity of the wall.
This is the principle used in the Pitot sensor to measure airspeed: